Iran shipped more than 25,000 pounds of enriched uranium to Russia on Monday, a key commitment in the nuclear deal that had be fulfilled before international sanctions can be lifted.

Secretary of State John F. Kerry applauded the transfer, calling it a “significant milestone” toward the deal’s implementation. He said the one shipment alone more than triples the estimated two- to three-month “breakout time” needed for Iran to acquire enough weapons-grade uranium to build one nucl...